The invention relates to a device for operating an electric motor of the type having means for detecting a current flowing through the electric motor, and having an evaluation arrangement which evaluates at least one characteristic of the current, compares it with a threshold value and outputs an overload signal in the event of overshooting of the threshold value. German Patent 28 20 330 discloses a circuit arrangement for an electric window lift drive which has an anti-jamming device. An evaluation arrangement detects a current flowing through an electric motor, and a temporal current change referred to time. The two characteristics determined for the current flowing through the electric motor are added and compared with a prescribed threshold value. After an overshoot of the threshold value, the electric motor is, for example, reversed, so that an object possibly jammed by a pane is released again. The evaluation arrangement contains a plurality of capacitors which implement low-pass filters with the aid of further components in each case. In addition to suppressing interference signals, the suppression of higher-frequency signal components in the determination of the characteristics also effects suppression of the detection of the starting operation, in the case of which a high motor current occurs which can be a multiple above the value of the steady-state motor current.
EP-A 0 116 904 discloses a drive device for a flap in air conditioning systems of motor vehicles. The flap is actuated by a stepping motor. It is ensured that the flap actuated by the stepping motor reliably reaches mechanical end stops due to the fact that, on the one hand, independently of the position of the flap which is to be prescribed the control device applies to the stepping motor at the beginning of an operating phase a maximum number of steps which suffices for one of the end stops of the flap to be reached. On the other hand, in the case of each further setting of the flap in each of the mechanical end stops, in addition to the steps required for this purpose the stepping motor has further steps applied to it in the direction of movement of the flap, in order reliably to reach the respective end stop of the flap owing to this number of extra steps, and to compensate for possible lost steps of the stepping motor.
It is the object of the invention to provide a device for operating an electric motor, which uses simple means to detect an overload or a blocked state of the electric motor, in particular a stepping motor.